In manufacturing microelectronic devices or microsystems, for instance, micromechanical devices, structures are formed in layers or substrates by lithographic processes. In such processes, usually, at first, resist patterns are formed in suitable resist materials and the resist patterns are transferred subsequently into a layer or a substrate using appropriate methods. Among others, photolithographic exposure processes are used as lithographic processes, wherein patterns formed in a mask are projected onto the resist material. In the course of further miniaturization of structures, exposure processes with extremely short wavelengths are used, for instance, extreme ultraviolet (“EUV”) lithography processes use a wavelength of 13.6 nm. The EUV masks used are mostly reflective masks because virtually all materials strongly absorb radiation in this wavelength range. EUV masks typically comprise a substrate with a reflective multilayer disposed on it and a patterned masking layer disposed on the multilayer.
However, defects may arise during the manufacturing of EUV blanks or substrates or of the EUV masks that, in turn, cause defects during the exposure of resist materials. Examples of such defects are, for instance, contaminations on the surface of the mask, defects in the patterns of the masking layer or defects within or beneath the multilayer.
Defects within or beneath the multilayer usually are called multilayer defects. They may, for instance, comprise particles, which are disposed beneath the multilayer or which are enclosed within the multilayer, deformations of the multilayer due to bumps or pits on the substrate surface, like scratches, or local variations of the layer thickness or the roughness of single layers of the multilayer. These defects may cause a variation of the amplitude or the phase of a radiation reflected by the multilayer. The resulting phase difference in reference to the radiation reflected by portions without defects may cause variations of the intensity of the reflected radiation due to interferences.